fix: live YouTube search 500 on the scrape source (published_at NOT NULL)

The scrape source exposes no publish date, so its search stub carried
published_at=None. Migration 0068 (PERF-15, E2/0.68.0) made
videos.published_at NOT NULL, so insert_stubs raised IntegrityError —
outside the route's page-fetch try/except — and the whole
/api/search/youtube request 500'd ("YouTube search failed."). Every
scrape search (the default source) had been broken since 0.68.0.

- routes/search._ingest_candidates: stub published_at falls back to
  now() (mirrors stub_from_playlist_item), satisfying the constraint.
- sync.videos.apply_video_details: prefer the enrich snippet's real
  date whenever present, so the now() placeholder is corrected instead
  of becoming the video's permanent date (the guard only filled a NULL,
  which a non-null placeholder no longer is). Never clobbers a real
  date with now().

Regression tests (DB lane): dateless scrape stub ingests and takes the
real enrich date; an enrich miss keeps a non-null placeholder, unkept.

backend/app/routes/search.py

@@ -136,6 +136,10 @@ def _ingest_candidates(
 
     # 2) Video stubs, flagged via_search. Existing rows keep their flags (ON CONFLICT DO
     #    NOTHING), so an organically-synced video stays organic.
+    #    published_at is NOT NULL (migration 0068); the scrape source exposes no publish date
+    #    (only a relative "3 years ago"), so fall back to now() to satisfy the constraint — the
+    #    enrich in step 3 overwrites it with the video's real snippet date.
+    now = datetime.now(timezone.utc)
     insert_stubs(
         db,
         [
@@ -143,7 +147,7 @@ def _ingest_candidates(
                 "id": it["id"],
                 "channel_id": it["channel_id"],
                 "title": it["title"],
-                "published_at": parse_dt(it["published_at"]),
+                "published_at": parse_dt(it["published_at"]) or now,
                 "thumbnail_url": it["thumbnail_url"],
                 "via_search": True,
             }
@@ -158,7 +162,6 @@ def _ingest_candidates(
             detail_by_id = {it["id"]: it for it in yt.get_videos(ids)}
     except YouTubeError:
         detail_by_id = {}
-    now = datetime.now(timezone.utc)
     for v in videos:
         item = detail_by_id.get(v.id)
         if item is None: